Modern spacecraft may be required to point more precisely than in the past, in order to accomplish their missions. Often, the reason for maintaining high pointing accuracy is so that a payload, such as an antenna or an optical instrument, may be accurately directed toward a particular feature or location.
FIG. 1 is a simplified perspective or isometric view of a spacecraft 8 orbiting the Earth 12. Spacecraft 8 includes a body 10, which supports an attitude sensor 16. Attitude sensor 16 generates signals representative of at least the pitch (y) attitude of body 10. Spacecraft body 10 carries a reaction wheel 46, which is mechanically driven by a motor 42.
FIG. 2 is a simplified block diagram of a control system 210, which controls motor 42, and thereby reaction wheel 46, in response to attitude signals from sensor 16 and attitude command signals from a source (not illustrated). Elements of FIG. 2 corresponding to those of FIG. 1 are designated by like reference numbers. In FIG. 2, attitude commands are applied over a signal path 212 to the non-inverting (+) input port of a subtracting circuit 214. Sensed attitude signals are applied from attitude sensor 16, by way of a signal path 218, to an inverting (-) input port of subtracting circuit 214. Attitude error signals are produced at the output of subtracting circuit 214, and are applied to a compensation circuit illustrated as a block 220. Compensation block 220 acts on the attitude error signals in a manner known in the art. One simple form of compensation is multiplication by a gain factor K.sub.1. But other forms of compensation are well known. The compensated attitude error signals at the output of compensation block 220 are applied by way of a path 222 to a non-inverting input port of a summing circuit 224, and to an ideal reaction wheel simulator 226. Summing circuit 224 converts the compensated attitude error signals into speed error signals, as described in more detail below.
The speed error signals at the output of 224 of FIG. 2 are applied by way of a path 236 to a reaction wheel driving block 238. Block 238 includes a driver illustrated as an amplifier 240, and also includes motor 42. Motor 42 is mechanically connected to reaction wheel 46, as described in conjunction with FIG. 1, and as suggested by path 244 in FIG. 2.
Reaction wheel 46 is torqued by motor 42 in response to speed error signals applied by block 238, and effects spacecraft body torquing. The spacecraft body torquing in turn, affects the spacecraft body attitude, which changes the attitude sensed by sensor 16, in such a manner as to tend to reduce the attitude error.
Imperfections in reaction wheel 46 such as electrical and mechanical losses, tend to cause the wheel speed errors to deviate from the commanded wheel speed. The wheel speed errors, in turn, result in incorrect body torques and consequent attitude errors. As described in U.S. Pat. No. 5,201,833, issued Apr. 13, 1993 in the name of Goodzeit et al., the reaction wheel speed is corrected by controls associated with an ideal wheel simulator. In FIG. 2, compensated attitude error signals are applied to a ideal wheel simulator 226, which produces ideal wheel speed signals on a path 228. In general, reaction wheel speed, as measured by digital speed sensor 248, is compared in a subtractor 230 with the ideal wheel speed signals and the differences between the actual and ideal wheel speeds is summed in a summer 224 with the compensated attitude error signals. More particularly, the ideal wheel speed signals produced by simulator 226 are applied to the noninverting input port of subtractor 230, and digital speed signals responsive to the actual reaction wheel speed are applied to its inverting input port. The resulting wheel speed difference signal (an error signal), representing the difference between the actual wheel speed and what it "should" be, are applied by way of a path 231 and a compensation network, illustrated as a block 232, and a path 233, to an input port of summing circuit 224. The wheel speed error signals are summed with the attitude error signals to produce the reaction wheel drive signals on signal path 236. The presence of the ideal wheel simulator changes the attitude error signals into reaction wheel speed error signals in summing circuit 224.
Most modern spacecraft attitude control systems are implemented by means of digital hardware and software. Hardware restrictions preclude the quantization (step size) of the digital speed sensor (248) from being as small as the quantization step size of other portions of the attitude control system. It has been discovered that, as the reaction wheel speed decreases toward zero, the effective size of the quantized speed signals produced by the digital speed sensor (248) becomes proportionally larger. Thus, as wheel speeds approach zero wheel speed, the sensed wheel speed increasingly deviates from the actual value. As a result, the difference signal produced by subtracting circuit 230 on signal path 231 is the difference between two numbers, one of which may be grossly in error. Thus, the difference itself may also be grossly in error. The effects of such errors is a tendency toward excessive reaction wheel motor heating, and control loop instability. These effects are undesirable. An improved spacecraft attitude control system is desired.